From DE-A1 3706 514, a method is known to determine, in a vehicle with a two-wheel drive, the reference speed for the development of a slip signal, other than by the control, through the speed of the slower of the non-driven wheels. In the event of instability, the increase of an auxiliary reference speed, which is determined, other than by the control, through the speed of the faster of the non-driven wheels, determines the reference speed.